This paper reviews the present understanding of the effects of metallurgical variables (composition, mechanical and thermal history, state of strain, etc.) on the superconducting properties of metals, alloys, and intermetallic compounds. The discovery of the persistence of superconductivity to high magnetic fields in 1961 initiated a revolution in the understanding of superconductivity. Previously, any materials departing from the simple behavior of “soft” or Type I superconductors were called “hard” superconductors, and their properties, although sometimes characterized, were never explained or exploited. We now realize that some of this “hard” behavior resulted from Type II superconductivity, some from a superconducting surface sheath, and some from specimen inhomogeneities. These concepts are reviewed briefly, and the effects of metallurgical variables on superconducting properties are explained in terms of these concepts where possible. The extreme structure sensitivity of superconducting properties has illuminated the need for further understanding of structure-property relations in superconductors and has given the metallurgist a unique opportunity to contribute both to the understanding and to the economic use of superconducting materials.